Elevator damper system



May ll, 1954 4 J. K. NoRTHRoP ETAL 2,678,176

ELEVATOR DAMPER SYSTEM Filed Jan. 29, 1952 2 Sheets-Sheet l N @MM May 11, 1954 J. K. NoRTHRoP ET AL `2,678,l76

ELEVATOR DAMPER SYSTEM Filed Jan. 29, 1952 2 Sheets-Sheet 2 @www Patented May 11, 1954 UNITED STATES PATENT OFFICE 2,67 8,17 (iV ELEVATOR DAMPER SYSTEM John K. Northrop, Pacific Palisades, and Warde L. Parker and Thomas A. Feeney, Los Angeles, Calif., assignors to Northrop Aircraft, Inc., Hawthorne, Calif., a corporation of California Application January 29, 1952, Serial No. 268,722

14 Claims. (Cl. 244-75) This invention relates to airplanes and more particularly to means for damping and preventing flutter of movable control surfaces on highspeed airplanes.

In high-speed airplanes having thin airfoils and` control surfaces subjected to high air loads, wherein space does not permit installation of the required actuating mechanism within the outlines of the surface, vibrations can cause structural deflections which have been known to reach destructive magnitudes, This is particularly true with any unbalanced` control surface, i. e., not mass-balanced about the hinge line. The flutter problem arises as a result of self-excited vibrations which start structural oscillations of relatively high frequencies and finally expand to literally shake olf a critically affected cantilevered part, for example.

Accordingly, it is an object of the present invention. to provide a flutter damper installation in an airplane to prevent the building up .of flutter of` a control surfaceabout its hinged attachment, at all speeds and conditions of flight.

It is known `that certain damper units have been devised for the` purpose .of `damping out vibrations of a surface or part hinged to a more solid structure.' The damper comprises one member to be held in some manner by the relatively solid structure and another member to be held by or mounted on the hinged surface. The

damper unit thus 4extends across the joint or gap from the so-called structural part to lthe movablepart so that internally restricted rela--` tive movement of the damper members occurs with deflection of the hinged part.

,lf the damper unit could be accurately aligned with respect to the hinge axis, the problem would` be solved. However, even with the most accurate` alignment practically and economically obtainabile, absolute coaxial rotation of the hinged surface and the damper cannot be achieved, thus resulting in binding of the associated elements to su'ch an extentthat the hinged surface freezes up and becomes immovable. Even if absolute jamming of the surface and damper units does not occur, the operational loads and stresses in the affected structure at deflected positions of the movable surface are excessive.

It is,` therefore,` another object ofthis inventionto provide a flutter damper installation for an airplane control surface wherein the surface can be swung through its entire range without binding of any kind due to misalignment of the damper `center of rotation withV the center line of the surface; hinge,while at the same time minimizing lost motion before action of the damper. A further object is to provide a failsafe means which will allow operation of the surface in the event of failure of a damper unit.

Our invention will be more fully understood by reference to the following detailed description of a specific embodiment thereof, and to the accompanying drawings.

In the drawings:

Figure 1 is a perspective view of an airplane having the elevator surfaces provided with a flutter damper installation according to the present invention.

Figure 2 is a perspective view of the elevator left side, showing hinge points, damper locations, and elevator control actuator.

Figure 3 is a cross-sectional view of the elevator and part of the horizontal stabilizer at one of the damper units, showing attachment of the damper unit, and taken as indicated by the line 3 3 in Figure 2.

Figure 4 is a view looking forward at a damper arm attachment fitting in the rear of the horizontal stabilizer.

Figure 5 is a perspective View of one end of a damper unit, showing the damper lever arm attachment to the unit and in the forward fitting.

Referring to Figure l and 2, an airplane has a horizontal stabilizer 2 and an elevator surface i hinged to the stabilizer 2 at three points `on each side'of the airplane center line, along an elevator hinge line 5. The elevator actuating components are located within the airplane structure near the centerline, and comprise an actuating lever l5 lined to rotate with the elevator l about the hinge line 5, and a hydraulic actuating cylinder assembly 'i pivotally connected at to the lever 6. The system shown is a full power system wherein a piston rod El of the cylinder assembly is pivotally connected to a stationary bracket Ill, the piston rod 9 having the customary piston sa attached to one end thereof and .operating within the cylinder. The housing of the cylinder l moves with the elevator 4 and has a control valve casing Il attached rigidly to the cylinder i. A valve control rod l2 operates valving within'the valve casing ll and extends out parallel to the piston rod 9 to pivotally connect to a cable-operated control quadrant i4 which is also rotatable in the bracket lll fixed to aircraft structure. Hydraulic pressure and return lines l 5 for operation of the actuating cylinder assembly 'i are supplied through the valve casing H.

Thus. movement of the quadrant it by cables ES will displace the valve rod l2 to cause deection of the elevator 4 to a new postiondependent j present.V f .Y

invention solvesthe; damper vsysj tem4 problemsby'. several meansM AsY shown i114, Figure`2the',responsibility-or damping is .di-- videdrbetyveenthe six damper unitsl fithree on eachv 'side 'of theY airplanecenter line). s'o-that fail-.lj ure of any one damper4 will'not seriously .caect i Y upon the amount of quadrant rotation at which new position the cylinder 'i and valve casing ll will have moved relative to the valve rod I2 to close the valve again and halt the elevator 4.

The elevator i in a typical high-speed airplane embodiment is only about 2.75 inches thick at the leading edgeso .thatI it isyery difficulttomountl any actual actuating mechanism inside` the ele-' vator l or stabilizer 2 directly at the hinge points.

Therefore, the elevator i becomes less rigid out-1;.

vvardly from the airplane center line to the elevator tips, and more susceptible to flutterfA To the rear of the hinge line. ,5 inthe elevator` 4, a main spar il (Figure 3)ljextendsfthellength To the spain llareixed hinge"v of the elevator.

fittings ii? which are boredlat -vtheehingefliney to receive hinge bolts, after being aligned in mounting position with respect@toradditionar'- hinge 'fittings 253 (Figure 4) projecting rearwardly y from the stabilizer 2.V Closely adjacent each elevatorhingeL fitting i9, a damper unit .2l is ser..

curely wmounted-on' thegrnain spar Il by attach.-

ment nuts 22' and-,spar studs 23,"as shown in Fig-.i

ures 2 and 3. The dempers are installed within' theconfines of the 'elevator surface.v 'Each damp.-

er unit 2i comprises a housing 12,5 vand a forwardly projecting arm "25 movable lrelative to the housing 24. The arm 25 rotates vvitha damper shaft 2E positioned with its longitudinal axis substantiallyjcoincident with the elevator hinge line 5.

The damper units '2 l' areofthe hydraulic vane type having'a predetermined restrictionthrough which a fluid must be forced in order to rotate theV shaft 26 in either direction.V The vdampers themselves form no partof the present invention and .Willnot be described in detail.

When the, ,elevator ..4 ,isinstalled the. basic damper principle is to retainthe arm 25 inthe stabilizer 2 vso that .deection. of` the velevaton turns the damper housing Zllbutnot the arm 25..V

Vibrations willthusbe damped andrapid'oscilr lations ofthe .elevator 4 prevented, dueto the Y hydraulic' restrictive action. However, ,We have found that if the damper arm 25 isjmerely'pinned or rigidly attached Vto the' stabilizer 2,101. example,

with the axis of the shaftjtfevenslightly displaced from the elevator hingeY line 5, deflections of the elevator l willcauseexcessive stressesrin the, structure and parts .making up -the loop consisting `of, the hinge fittings, stabilizer and elevator structure, and damper linkage acrossjshe hingerline 5. These-,stresses seriously impairope eratonnitheV eievatoriand Vare often suiiicient to.

causeeompletaiamming insidegof ,the` damper unit/.2li

different axes, caused by manuacturing tolera i'hisisdua of course,to thejrotation oitheelevator il `and damper `unit 2l' about tivov ances andsrnall structural deflections. :Even ify 1 thedamper sha'itis successfullyl aligned with thehingeline 5 atone .elevator position, actuation of i.the elevatorto. other positions. and backv again,

loadsunfallfconnection pointsin reverse directions; and since some,.t.\aollashY is bound to bei.

Present,.thisrreversal of loadsresults .inldisturbr tation., 'Ihejproblemof binding `is .therefore The. present operation ofitheairplane. /.,The .dempers arelo cated..just` beside .the hingepoints, vvhclnare` thebest points for keiiic'ient absorption of `flutter Vingthe relative positions of the twoaxes `of. r'ostill a vertical direction. The head 2! fits into an arm tting 3Q mounted within the stabilizer 2.

Thearmitting 3l) is attached to a shear ,web

3iA in Athe,stabilizer and comprises a fouruzlationV members?! containing a rearward opening 34 bounded atthe bottom by a attcpped bushing 35 and lat thetopbyfa smooth-ended adjustment bolt 3G installeL in a threaded insert 3'! in the member-e2!- A-retaining element 3S holds the insert 3i from turning, and the insert has locking grooves til by which the insert may be compressed before assembly, to lock, the position of theade justment'bolt 36.' Surfaces such as the.l bearing surfaces'29", the, top :of the bushing 35, andthe bottom of-the bolt 36are preferablycaseharde ened..

Whentheelevator 4 is installed, the head 21 of the arm '25 nts into the opening 34' with the..

bearing surfaces 2) directly between the bushing 35and theend of the boltr 35.* The bearing rsurrv i"'aces 29 are made sufficiently short inthe fore.-

andaftdirection socthat misalignmentof compo,.-V nents inthe vertical direction will not cause arm Y binding with elevator rotation. ProperY bolt afl-.

justement-is attained by making'the clearance` bec.-

, tween vthe head 2l andthetting. suriacessmall enough vso that `practically A no measurable lost,..-

motion of the head ,21 in a vertical direction ocr,-

curs, but large enough" 'so that the head1-Can easily slide in a fore-and-.aft direction as needed.

`Anactual value ofthisclose-fit clearance, forexf ample, is about-.001 :inch: l.,

Y It is thusseenthat,'inoperation ofthe elevator, fi, anyV misalignment of the, damper shaft'centerv line and the actual line of the elevator hinge during-'rotationY Williresult in Va slight.foreandaft translationtof the arm 25 relative to the arm fit1 ting at. Therefore, this action prevents -any-4 and. all binding or excessive stresses at the hinge,points,

andin the `damper 'units'. ,2 l.

The. lost` motion... of the complete installation `before relative move-.,. Y

ment'. of the.. damper componentsoccursV VVand.

damping actionstarts, isonly .06 of elevator .ro.

tation, with vthe arm clearanceadjustment aspre-`v Y viously described; Thisangleis considerably less.;` than that which Wouldoccur as free motion due-.

-to backlashLinitheelevator connections. if the Y daznpers ,were omitted,...Th'eLdamper system icane.V thusilimit,oscillationsiof theelevator.. 4 to va .very

small..amplitudalfallingrfar .short of any..dan.` gerous flutter. Sinceit would.beldiiculttofpro-e.. Jvideenough. forcetostopany hghwdegree flutter after.` it had `started, ,the objectAA is. to preventthe..` 1

undesirable. situation vrfrom:.starting, whichY theA presentinvention doesn.V Y Y Vfis-a specic .exampleot therequirementsI of ,the v'cla-:innerer,units :2 i each r damper .-this;;deserib.ed-. embodiment `givesaa torque. coecient:.of -more f than pound-feetvforeach radian persecondof 'lhsrcoeicientwrnust de demonstrated :under test cond-itionsof plus and minus.0;251 amplitude from,"

cycles per minute:

' a giveny neutralY position and1 a frequency fof'f3000i The Asafetyfeature'mentionedfhereinbeforefis bestshownin"Figures ..3 and 5. g The arm "'25'fis to a collar 4| of the damperl unit 2| by two special size rivets 42. The collar 4| is solidly fixed to the shaft 26, with no backlash. The rivets 42 are designed to be the weakest members of the damper installation, and they will shear off at a torque load of between 300 and 490 pound-feet, for example. Thus, if the damper unit 2| jams internally, the rivets 42 will break at their designed load, and the arm 25 will rotate freely on the co1- lar 4| to insure further elevator control movement.

From the above description, it is readily seen that a flutter damper installation which will attain the objects set forth has been provided. The damper system of the'present invention is especially suited for use with a full power surface control sysetem, because of the high operating torque required to move the surface. With the present invention, weight can be saved by eliminating the usual heavy balancing structure ahead of the surface hinge line, and leaving the surface entirely unbalanced.

While in order to comply with the statute, the invention has been described in language more or less specific as to structural features, it is to be understood that the invention is not limited to the specific features shown, but that the means and construction herein disclosed comprise the preferred form of putting the invention into effect, and the invention is therefore claimed in any of its forms or modifications within the legitimate and valid scope of the appended claims.

What is claimed is:

l. In an airplane having a movable surface member hinged to another member along a hinge line, a flutter damper system comprising a plurality of damper units each having two coaxially rotatable elements wherein oscillations of said elements are damped, each of said damper units being mounted with its axis of relative coaxial rotation on said hinge line, one of said damper elements mounted rigidly on one of said members, and the other element slidably mounted in the other member, the direction of allowable sliding being along a line substantially intersecting said hinge line at a right angle, and means confining said other element from circumferential movement about said hinge line.

2. In an airplane having a movable surface member hinged to another member along a hinge line, a flutter damper system comprising a plurality of damper units each having a base element and an arm having dampedrotation relative to said base element, said damper units mounted with the rotational axis of said arm on said hinge line, said base element being rigidly attached to one of said members, arm mounting means xed to the other of said members in position to receive the outer end of said arm, said mounting means having bearing surfaces to` just Contact opposite sides of said arm end to prevent said arm from moving relative to said mounting means in a circumferential direction about said rotational axis, said arm resting between said bearing surfaces with no obstructions to prevent sliding of said arm in a line substantially intersecting said rotational axis and said hinge line at a right angle.

3. Apparatus in accordance with claim 2 wherein one of said bearing surfaces is adjustably related to said mounting means to vary the clearv in a' direction toward the other of said bearing surfaces.

5. Apparatus in accordance with claim 2 wherein each of said arms is fastened to its respective damper unit by easily replaceable means having a designed working strength below that of any other part in said damper unit and above that required for normal operational loads, whereby in the event of excessive binding or jamming of said damper unit, said replaceable means will break and allow free rotation of said arm relative to said base element.

6. In an airplane having an unbalanced hinged control surface actuated about its hinge line by a full power control system, means for preventing flutter of said surface comprising a utter damper unit having two members with damped relative rotation therebetween mounted with its axis of rotation on said hinge line, 'one of said members attached rigidly to said surface, and the other member slidably fitting into the part to which said surface is hinged, the direction of sliding being along a line substantially intersecting said hinge line at a right angle, and means for confining said other member from circumferential movement about said hinge line, whereby deflection of said member is equal to the deflection of said surface, and whereby no stresses are incurred as a result of misalignments of said damper rotational axis with said hinge line.

7. In an airplane, the combination of an unbalanced control surface member hinged to another member of said airplane along a hinge line, full power control means for actuating said surface about said hinge line, a flutter damper unit having two elements with damped relative rotation therebetween mounted with its axis of rotation on said hinge line, one of said elements attached rigidly to one of said members, and the other element slidably fitting into the other member, the direction of allowable sliding being along a line substantially intersecting said hinge line at a right angle, and means attached to said other member for confining said other element against circumferential motion about said hinge line.

8. Apparatus in accordance with claim 7 wherein one of said damper elements contains easily replaceable joining means having a designed strength below that of any other part in said damper unit in the direction of resistance encountered when said elements are relatively operated, and above that required during normal surface operation, whereby said replaceable means will be the component which breaks first in case of jamming of said damper unit, 'to allow unrestricted relative rotation of said damper elements.

9. In an airplane, the combination of an unbalanced control surface member hinged at a plurality of hinge points to another member of said airplane along a hinge line, full power control means for actuating said surface about said hinge line, an equal plurality of flutter damper units each having two elements with damped relative rotation therebetween, one of said damper units located closely adjacent each of said hinge points, respectively, with the rotational axes of said damper units on said hinge line, one of said damper elements of each unit attached rigidly to one of said members, the other element of each damper unit slidably tting into the other member, the direction of allowable sliding being along a line substantially intersecting,- saic hinge mie-at a, right, angie,v andi means attached to said other member for con! ning; saidi other element of each'` damper unit aga-inst: circumferential motiont about said hi-nge line;

1.0. In an airplane, the combination of anunbalanced control; surface hinged at a plurality of hinge points to a, member of said airplane along a hingef line, an equal plurality of flutter dan'iper.v units each:v havingia base element and air-arm havingy damped rotation relative, to said base element, one of said damper unitsi located closely adjacent, each ofsaid hinge points, respectivel-y',` with the; rotational axes ofsaid arms on'fsa-id hingef line, said base element of each damper unit being' rig-idlyattached: to saidv control surface, a plurality of arm mounting means fixed to said airplane member in positiony to receive the outer endsof said arms; respectively, each of said mounting means having bearing surfaces` tojust contact opposite sides of its respective arm toprevent. said arms from movingA relativeto saidrnounting means a circumferentialr direction about said hinge line; each of said arms resting between said' bearing surfaces with no obstructions to preventsliding of? said arm ina linesubstantially.v intersecting said hinge-line ata right angle.

Y 11.` In a. high-speed airplane having1 thin airfoi-ls, thecombina-tion or a control surface member .hinged at a plurality of hinge points to another membery of4 said. airplane along a hinge line, aifull power control system for operating said surface` member and having actuator means connectedV to said surfaceA member near the center'ot thespan thereof, only, an equal plurality ofiiutter damper units each having. a housing element' and; an armY having damped rotation; relative to, said housing element, one-of said damper units locatedl closely adjacentY each of said hinge points, respectively, with; the rota,- tonal axes of said arms on said hinge line, said housingY element of each damperV unit being rigidly attached toY one of said members, af plurality of arm fitting means attached, to-'the other of said: members inY position to receive the outer ends of said arms, respectively, the outer end of each arm havingV a head. por-tion,Y with two opposite rela-tively short plane parallel surf-aces on the radially` spaced sides'of said head, each of Vsaid fitting means having bearing.- surfaces spaced to just contact theY short surfaces of, its respective arml head to prevent said.y armf from moving relativeA to said, fitting means infa; circumferential direction. about said'hinge-line, each of, said arm heads resting betweenl saidbearing surfaces with Vno obstructions' tov prevent: sliding ci said arm in a linevsubstantiallyV intersecting said hinge line at a right angle.

1:2, Apparatus in accordance with claimv 11 wherein one of saidbearing surfaces is adjust,-

amy mauntecrinsaid fraingmeans to. vary mey 13. In an airplanerhaving a. flutter damper uni-t Y of the rotational type mounted' on the hinge line of a movable surfacetoprevent build-up of vibratory oscillations; of said surface; means for eliminating excessive stresses and binding of parts due to misalignment of the damper units axis and said hinge line, comprising an arm attached to and forming.V the sole externa-l connection of one otheflrotationally damped members of said unit, sa-id arm extending in a, radial direction from the operating axis of said unit, the other damped member ofA said unit secured rigidly to said sur,- face, arm iitting mea-ns attached: to a member of said airplane upon which said surface is hinged, in: position to slidably receive said arm, said tting means having bearing surfaces spaced to just contact opposite sides, of said arm to prevent said,V arm from moving relativeto said tting means in a circumferential direction about said hinge line, said armresting between said bearingsurfaces with no obstructions which would prevent sliding of said armin a line substantially intersectingA said hinge line at a. right angle.

lli In, an airplanehaving a utter damper unit of the rotational type mounted with its operational axisv on. thefhingeline of a movable surface member hinged to an airplanes member, said damper being attached rigidly to one of said members and having a damped element to be connected tol the other member, connectionmeans comprising an arm xed to said damped element and extending in a radial direction. from said axis, two opposite relatively short plane parallel surfaces on the radially spaced sides of said arm near the outer end thereof, and arm fitting means attached to said other member in position to receive the outer end of said arm, said fitting meansA having bearing surfaces spaced to just contact the short surfaces of said arm to prevent said arm from moving relative to said tting means in a circumferential direction about said hinge line, and said arm resting between said. bearingv surfaces withino impediments which would prevent Vsliding oi said arm in a line subst'antially intersecting said hinge line at alright angle, whereby no binding stresses occur as a result ofr slightmisalignments of said damper unit axis, andk said hinge line.

References Cited inthe le ofV this patent Y UNITED STATES PATENTSv Number Name Date i 1;,45832518` De Brettevilleet al. JuneY 12, 1923 137295210 Delage Sept; 24, 1929- 

